U.S. patent application number 13/480079 was filed with the patent office on 2012-12-27 for high speed optical sensor inspection system.
Invention is credited to Hiroshi Anzai, Beverly Caruso, Steven K. Case, Chuanqi Chen, Carl E. Haugan, Todd D. Liberty, Timothy A. Skunes.
Application Number | 20120327215 13/480079 |
Document ID | / |
Family ID | 47361477 |
Filed Date | 2012-12-27 |
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United States Patent
Application |
20120327215 |
Kind Code |
A1 |
Case; Steven K. ; et
al. |
December 27, 2012 |
HIGH SPEED OPTICAL SENSOR INSPECTION SYSTEM
Abstract
An optical inspection sensor is provided. The sensor includes an
array of cameras configured to acquire image data relative to a
workpiece that moves relative to the array of cameras in a non-stop
fashion. An illumination system is disposed to provide a pulse of
illumination when the array of cameras acquires the image data. At
least some image data includes data regarding a skip mark or
barcode on the workpiece.
Inventors: |
Case; Steven K.; (St. Louis
Park, MN) ; Caruso; Beverly; (St. Louis Park, MN)
; Liberty; Todd D.; (Apple Valley, MN) ; Skunes;
Timothy A.; (Mahtomedi, MN) ; Haugan; Carl E.;
(St. Paul, MN) ; Chen; Chuanqi; (Singapore,
SG) ; Anzai; Hiroshi; (Kanagawa, JP) |
Family ID: |
47361477 |
Appl. No.: |
13/480079 |
Filed: |
May 24, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12940214 |
Nov 5, 2010 |
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13480079 |
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12886803 |
Sep 21, 2010 |
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12940214 |
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61492093 |
Jun 1, 2011 |
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61244616 |
Sep 22, 2009 |
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61244671 |
Sep 22, 2009 |
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Current U.S.
Class: |
348/92 ; 235/375;
348/E7.085 |
Current CPC
Class: |
H05K 13/0815 20180801;
G06K 9/2036 20130101; G06K 2009/2045 20130101; G01N 21/956
20130101; G01N 21/8806 20130101; G01N 21/8903 20130101 |
Class at
Publication: |
348/92 ; 235/375;
348/E07.085 |
International
Class: |
H04N 7/18 20060101
H04N007/18; G06F 17/00 20060101 G06F017/00 |
Claims
1. An electronics assembly system comprising: an optical inspection
sensor including: an array of cameras configured to acquire image
data relative to a workpiece that moves relative to the array of
cameras in a non-stop fashion; an illumination system disposed to
provide a pulse of illumination when the array of cameras acquires
the image data; and wherein an least some image data includes data
regarding a skip mark on the workpiece; a component placement
machine configured to receive the data regarding the skip mark and
to selectively place components on the workpiece based on the
data.
2. The electronics assembly system of claim 1, and further
comprising a processor configured to receive the image data and
provide an indication regarding the presence of the skip mark to
the electronics assembly machine.
3. An optical inspection sensor comprising: an array of cameras
configured to acquire image data relative to a workpiece that moves
relative to the camera in a first dimension; wherein the array of
cameras has an effective field of view that encompasses the entire
workpiece in a dimension transverse to the first dimension. an
illumination system disposed to provide a pulse of illumination
when the array of cameras acquires the image data; and a processor
configured to process the image data to identify and read a barcode
on the workpiece.
4. The optical inspection sensor of claim 3, wherein the barcode is
read by the sensor and information is provided by the sensor to a
component placement machine based on the barcode.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] The present application claims is based on and claims the
benefit of U.S. Provisional Application Ser. No. 61/492,093 filed
Jun. 1, 2011; and the present application is a Continuation-In-Part
application of U.S. patent application Ser. No. 12/940,214, filed
Nov. 5, 2010, which application is a Continuation-In-Part
application of U.S. patent application Ser. No. 12/886,803, filed
Sep. 21, 2010, which application is based on and claims the benefit
of U.S. Provisional Application Ser. No. 61/244,616, filed Sep. 22,
2009 and U.S. Provisional Application Ser. No. 61/244,671, filed on
Sep. 22, 2009.
COPYRIGHT RESERVATION
[0002] A portion of the disclosure of this patent document contains
material which is subject to copyright protection. The copyright
owner has no objection to the facsimile reproduction by anyone of
the patent document or the patent disclosure, as it appears in the
Patent and Trademark Office patent file or records, but otherwise
reserves all copyright rights whatsoever.
BACKGROUND
[0003] Automated electronics assembly machines are often used in
the manufacture of printed circuit boards, which are used in
various electronic devices. The manufacturing process is generally
required to operate quite swiftly. Rapid or high speed
manufacturing ensures that costs of the completed printed circuit
board are minimized. However, the speed with which printed circuit
boards are manufactured must be balanced by the acceptable level of
scrap or defects caused by the process. Printed circuit boards can
be extremely complicated and small and any one board may have a
vast number of components and consequently a vast number of
electrical connections. Printed circuit boards are now produced in
large quantities. Since such printed circuit boards can be quite
expensive and/or be used in expensive equipment, it is important
that they be produced accurately and with high quality, high
reliability, and minimum scrap. Unfortunately, because of the
manufacturing methods available, some level of scrap and rejects
still occurs. Typical faults on printed circuit boards include
inaccuracy of placement of components on the board, which might
mean that the components are not correctly electrically connected
in the board. Another typical fault occurs when an incorrect
component is placed at a given location on a circuit board.
Additionally, the component might simply be absent, or it may be
placed with incorrect electrical polarity. Further still, if there
are insufficient solder paste deposits, this can lead to poor
connections. Additionally, if there is too much solder paste, such
a condition can lead to short circuits, and so on. Further still,
other errors may prohibit, or otherwise inhibit, electrical
connections between one or more components, and the board. An
example of this condition is when a small, "stray" electrical
component is accidentally released onto a section of the circuit
board where another component is to be subsequently placed by
another placement operation. This stray component may prevent
electrical connectivity of the "correct" component that is placed
onto the printed circuit board after the stray component. The
condition if further exacerbated when the correct component has a
package style, such as a ball grid array (BGA) or flip chip, where
the electrical connections are visibly hidden after placement. In
this condition, the stray component and the integrity of the solder
joints cannot be visibly inspected either manually or by automated
optical inspection (AOI) systems for errors or defects since the
defects are hidden by the component package. X-ray systems may
detect these errors, but these systems remain too slow and
expensive for wide spread adoption in most printed circuit board
assembly lines.
[0004] Conventional automated optical inspection systems receive a
substrate, such as a printed circuit board, either immediately
after placement of the components upon the printed circuit board
and before wave soldering, or post reflow. Typically, the systems
include a conveyor that is adapted to move the substrate under test
through an optical field of view that acquires one or more images
and analyzes those images to automatically derive conclusions about
components on the substrate and/or the substrate itself. The amount
of time to initially program the inspection inputs is often high
for these systems and also to fine tune the inspection parameters
or models. Another drawback to these automated optical inspection
systems is that, although they can identify manufacturing errors,
they often provide little help to identify the particular processes
that caused the manufacturing error. As such, a need has arisen to
provide an improved inspection system that simplifies the initial
inspection programming as well as providing additional insight into
the root cause of manufacturing errors.
SUMMARY
[0005] An optical inspection sensor is provided. The sensor
includes an array of cameras configured to acquire image data
relative to a workpiece that moves relative to array of cameras in
a non-stop fashion. An illumination system is disposed to provide a
pulse of illumination when the array of cameras acquires the image
data. At least some image data includes data regarding a skip mark
or barcode on the workpiece.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] FIG. 1 is an elevation view of an automated high speed
optical inspection system with a camera array and compact,
integrated illuminator in accordance with an embodiment of the
present invention.
[0007] FIG. 2 is a diagrammatic elevation view of a plurality of
cameras having overlapping fields of view in accordance with an
embodiment of the present invention.
[0008] FIG. 3 is a system block diagram of an inspection system in
accordance with an embodiment of the present invention.
[0009] FIG. 4 is a top plan view of a transport conveyor, printed
circuit board, and a camera array field of view acquired with a
first illumination field type.
[0010] FIG. 5 is a top plan view of a transport conveyor, printed
circuit board, and a camera array field of view acquired with a
second illumination field type.
[0011] FIGS. 6A-6D illustrate a workpiece and camera array fields
of view acquired at different positions and under alternating first
and second illumination field types in accordance with an
embodiment of the present invention.
[0012] FIG. 7 is a block diagram of an exemplary printed circuit
board assembly line that includes an inspection system in
accordance with an embodiment of the present invention.
[0013] FIG. 8 is a front elevation view of a portion of an assembly
line.
[0014] FIG. 9A is a diagrammatic view of exemplary solder paste
deposits identified by an inspection program in accordance with an
embodiment of the present invention.
[0015] FIG. 9B is a diagrammatic view of an exemplary image of the
same region depicted in FIG. 9A captured with an optical inspection
sensor after an assembly operation in accordance with an embodiment
of the present invention.
[0016] FIG. 9C is a diagrammatic view of a difference image between
FIGS. 9A and 9B.
[0017] FIG. 10A is a diagrammatic view of an exemplary image
acquired by an optical inspection system in accordance with an
embodiment of the present invention.
[0018] FIG. 10B is a diagrammatic view of an exemplary image
acquired by an optical inspection sensor where a stray component
has been placed.
[0019] FIG. 10C is a diagrammatic difference image between FIGS.
10A and 10B.
DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
[0020] FIG. 1 shows an elevation view of a system for generating
high contrast, high speed digital images of a workpiece that are
suitable for automated inspection, in accordance with an embodiment
of the present invention. Camera array 4 consists of cameras 2A
through 2H preferably arranged at regular intervals. Each camera 2A
through 2H simultaneously images and digitizes a rectangular area
on a workpiece or substrate, such as printed circuit board 10,
while the workpiece undergoes relative movement with respect to
cameras 2A through 2H. Illuminator 45 provides a series of pulsed,
short duration illumination fields referred to as strobed
illumination. The short duration of each illumination field
effectively "freezes" the image of printed circuit board 10 to
suppress motion blurring. Two or more sets of images for each
location on printed circuit board 10 are generated by camera array
4 with different illumination field types for each exposure.
Depending on the particular features on printed circuit 10 board
that need to be inspected, the inspection results may be
appreciably enhanced by joint processing of the reflectance images
generated with different illumination field types. The different
illumination field types may include cloudy day, brightfield, or
darkfield, for example.
[0021] Workpiece transport conveyor 26 translates printed circuit
board 10 in the X direction in a nonstop mode to provide high speed
imaging of printed circuit board 10 by camera array 4. Conveyor 26
includes belts 14 which are driven by motor 18. Optional encoder 20
measures the position of the shaft of motor 18 hence the
approximate distance traveled by printed circuit board 10 can be
calculated. Other methods of measuring and encoding the distance
traveled of printed circuit board 10 include time-based, acoustic
or vision-based encoding methods. By using strobed illumination and
not bringing printed circuit board 10 to a stop, the time-consuming
transport steps of accelerating, decelerating, and settling prior
to imaging by camera array 4 are eliminated. It is believed that
the time required to entirely image a printed circuit board 10 of
dimensions 210 mm.times.310 mm can be reduced from 11 seconds to 4
seconds using embodiments of the present invention compared to
coming to a complete stop before imaging.
[0022] FIG. 2 shows the Y dimension location of each field of view
30A through 30H on printed circuit board 10 that is imaged by
cameras 2A through 2H, respectively. There is a slight overlap
between adjacent fields of view in order to completely image all
locations on printed circuit board 10. During the inspection
process, the images of discrete fields of view 30A through 30H are
digitally merged, or stitched, into one continuous image in the
overlap regions. Example camera array 4 is shown in FIGS. 1 and 2
arranged as a single dimensional array of discrete cameras. As
shown, cameras 2A-2H are configured to image in a non-telecentric
manner. This has the advantage that the fields of view 30A through
30H can be overlapped. However, the magnification, or effective
resolution, of a non-telecentric imaging system will change as
printed circuit 10 and its features are positioned closer or
further away from cameras 2A-2H. Effects of circuit board 10
warpage, thickness variations and other camera alignment errors can
be compensated by image stitching. In another embodiment, the
camera array may be arranged in a two dimensional array. For
example, the discrete cameras may be arranged into a camera array
of two columns of four cameras where adjacent fields of view
overlap. Other arrangements of the camera array may be advantageous
depending on cost, speed, and performance goals of the inspection
system, including arrays where the fields of view do not overlap.
For example, a staggered array of cameras with telecentric imaging
systems may be used.
[0023] FIG. 3 is a block diagram of inspection system 92.
Inspection application program 71 preferably executes on system
computer 76. Inputs into inspection program 71 include the type of
printed circuit board 10, CAD information describing the location
and types of components on printed circuit board 10, the features
on printed circuit board 10 to be inspected, lighting and camera
calibration data, the transport conveyor 26 direction, et cetera.
Inspection program 71 configures programmable logic controller 22
via conveyor interface 72 with the transport direction, velocity,
and width of printed circuit board 10. Inspection program 71 also
configures main electronics board 80 via PCI express interface with
the number of encoder 20 counts between each subsequent image
acquisition of camera array 4. Alternatively, a time-based image
acquisition sequence may be executed based on the known velocity of
printed circuit board 10. Inspection program 71 also programs or
otherwise sets appropriate configuration parameters into cameras
2A-2H prior to an inspection as well as strobe board 84 with the
individual flash lamp output levels.
[0024] Panel sensor 24 senses the edge of printed circuit board 10
as it is loaded into inspection system 92 and this signal is sent
to main board 80 to begin an image acquisition sequence. Main board
80 generates the appropriate signals to begin each image exposure
by camera array 4 and commands strobe board 84 to energize the
appropriate flash lamps 87 and 88 at the proper time. Strobe
monitor 86 senses a portion of light emitted by flash lamps 87 and
88 and this data may be used by main electronics board 80 to
compensate image data for slight flash lamp output variations.
Image memory 82 is provided and preferably contains enough capacity
to store all images generated for at least one printed circuit
board 10. For example, in one embodiment, each camera in the array
of cameras has a resolution of about 5 megapixels and memory 82 has
a capacity of about 2.0 gigabytes. Image data from cameras 2A-2H
may be transferred at high speed into image memory buffer 82 to
allow each camera to be quickly prepared for subsequent exposures.
This allows the printed circuit board 10 to be transported through
inspection system 92 in a nonstop manner and generate images of
each location on printed circuit board 10 with at least two
different illumination field types. The image data may begin to be
read out of image memory into PC memory over a high speed
electrical interface such as PCI Express (PCIe) as soon as the
first images are transferred to memory 82. Similarly, inspection
program 71 may begin to compute inspection results as soon as image
data is available in PC memory.
[0025] The image acquisition process will now be described in
further detail with respect to FIGS. 4-6.
[0026] FIG. 4 shows a top plan view of transport conveyor 26 and
printed circuit board 10. Cameras 2A-2H image overlapping fields of
view 30A-30H, respectively, to generate effective field of view 32
of camera array 4. Field of view 32 is acquired with a first
strobed illumination field type. Printed circuit board 10 is
transported by conveyor 26 in a nonstop manner in the X direction.
Printed circuit board 10 preferably travels at a velocity that
varies less than five percent during the image acquisition process,
although larger velocity variations and accelerations may be
accommodated.
[0027] In one preferred embodiment, each field of view 30A-30H has
approximately 5 million pixels with a pixel resolution of 17
microns and an extent of 33 mm in the X direction and 44 mm in the
Y direction. Each field of view 30A-30H overlaps neighboring fields
of view by approximately 4 mm in the Y direction so that
center-to-center spacing for each camera 2A-2H is 40 mm in the Y
direction. In this embodiment, camera array field of view 32 has a
large aspect ratio in the Y direction compared to the X direction
of approximately 10:1.
[0028] FIG. 5 shows printed circuit board 10 at a location
displaced in the positive X direction from its location in FIG. 4.
For example, printed circuit board may be advanced approximately 14
mm from its location in FIG. 4. Effective field of view 33 is
composed of overlapping fields of view 30A-30H and is acquired with
a second illumination field type.
[0029] FIGS. 6A-6D show a time sequence of camera array fields of
view 32-35 acquired with alternating first and second illumination
field types. It is understood that printed circuit board 10 is
traveling in the X direction in a nonstop fashion. FIG. 6A shows
printed circuit board 10 at one X location during image acquisition
for the entire printed circuit board 10. Field of view 32 is
acquired with a first strobed illumination field type as discussed
with respect to FIG. 4. FIG. 6B shows printed circuit board 10
displaced further in the X direction and field of view acquired
with a second strobed illumination field type as discussed with
respect to FIG. 5. FIG. 6C shows printed circuit board 10 displaced
further in the X direction and field of view 34 acquired with the
first illumination field type and FIG. 6D shows printed circuit
board 10 displaced further in the X direction and field of view 35
acquired with the second illumination field type.
[0030] There is a small overlap in the X dimension between field of
views 32 and 34 in order to have enough overlapping image
information in order to register and digitally merge, or stitch
together, the images that were acquired with the first illumination
field type. There is also small overlap in the X dimension between
field of views 33 and 35 in order to have enough overlapping image
information in order to register and digitally merge the images
that were acquired with the second illumination field type. In the
embodiment with fields of view 30A-30H having extents of 33 mm in
the X direction, it has been found that an approximate 5 mm overlap
in the X direction between field of views acquired with the same
illumination field type is effective. Further, an approximate 14 mm
displacement in the X direction between fields of view acquired
with different illumination types is preferred.
[0031] Images of each feature on printed circuit board 10 may be
acquired with more than two illumination field types by increasing
the number of fields of view collected and ensuring sufficient
image overlap in order to register and digitally merge, or stitch
together, images generated with like illumination field types.
Finally, the stitched images generated for each illumination type
may be registered with respect to each other. In a preferred
embodiment, workpiece transport conveyor 26 has lower positional
accuracy than the inspection requirements in order to reduce system
cost. For example, encoder 20 may have a resolution of 100 microns
and conveyor 26 may have positional accuracy of 0.5 mm or more.
Image stitching of fields of view in the X direction compensates
for positional errors of the circuit board 10.
[0032] The image contrast of various object features vary depending
on several factors including the feature geometry, color,
reflectance properties, and the angular spectrum of illumination
incident on each feature. Since each camera array field of view may
contain a wide variety of features with different illumination
requirements, embodiments of the present invention address this
challenge by imaging each feature and location on workpiece 10 two
or more times, with each of these images captured under different
illumination conditions and then stored into a digital memory. In
general, the inspection performance may be improved by using object
feature data from two or more images acquired with different
illumination field types.
[0033] It should be understood that embodiments of the present
invention are not limited to two lighting types such as dark field
and cloudy day illumination field nor are they limited to the
specific illuminator configurations. The light sources may project
directly onto workpiece 10. The light sources may also have
different wavelengths, or colors, and be located at different
angles with respect to workpiece 10. The light sources may be
positioned at various azimuthal angles around workpiece 10 to
provide illumination from different quadrants. The light sources
may be a multitude of high power LEDs that emit light pulses with
enough energy to "freeze" the motion of workpiece 10 and suppress
motion blurring in the images. Numerous other lighting
configurations are within the scope of the invention including
light sources that generate bright field illumination fields or
transmit through the substrate of workpiece 10 to backlight
features to be inspected.
[0034] Inspection performance may be further enhanced by the
acquisition of three dimensional image data. For example,
electrical component polarity marks such as notches, chamfers, and
dimples are three dimensional in nature. Acquisition of three
dimensional solder paste deposit image data enables measurement of
critical height and volume parameters. Further, three dimensional
image data can improve segmentation and identification of small
features with height relative to the nearly flat substrate.
[0035] Three dimensional information such as the profile of a
solder paste deposit may be measured using well known laser
triangulation, phase profilometry, or moire methods, for example.
U.S. Pat. No. 6,577,405 (Kranz, et al) assigned to the assignee of
the present invention describes a representative three dimensional
imaging system. Stereo vision based systems are also capable of
generating high speed three dimensional image data.
[0036] To acquire high speed two and three dimensional image data
to meet printed circuit board inspection requirements, multiple
camera arrays may be arranged in an angled, stereo configuration
with overlapping camera array fields of view. The circuit board can
then be moved in a nonstop fashion with respect to the camera
arrays. Multiple, strobed illumination fields effectively "freeze"
the image of the circuit to suppress motion blurring.
[0037] Application inspection program 71 computes three dimensional
image data by known stereo methods using the disparity or offset of
image features between the image data from the angled camera arrays
arranged in a stereo configuration.
[0038] FIG. 7 is a block diagram of example automated printed
circuit board assembly line 110 that includes an inspection system
in accordance with an embodiment of the present invention. Solder
paste screen printer 112 prints solder deposits in circuit board
10. A first, high throughput, component placement machine 114
places a number of electrical components on printed circuit board
10. Automated surface mount technology (SMT) assembly lines are
often configured with one or more high speed "chip shooter"
component placement machines that are optimized to place smaller
components such as chip resistors and capacitors at high throughput
rates. A second component placement machine 116 is illustrated and
is often configured to place a wider range of component styles and
sizes, albeit at slower throughput rates. For example, component
placement machine 116 may place electrical connectors, ball grid
array (BGA) components, flip chip components, quad flat pack (QFP)
components, as well as smaller passive electrical components on
circuit board 10. Reflow oven 118 melts the solder paste deposits
to create mechanical attachment and electrical connection of the
components to circuit board 10. Automated optical inspection system
120 provides final inspection of circuit board 10. Conveyors 122,
124, 126, and 128 transport circuit boards between various
automated assembly machines in assembly line 110. As used herein a
conveyor is intended to mean one or more automatic transport
systems that move a workpiece or substrate from one location to
another without human assistance. Moreover a conveyor may include
an input buffer where workpieces can aggregate prior to an assembly
operation. Thus, while a single conveyor 122 is shown coupling
screen printer 112 to placement machine 114, such illustration is
for clarity since conveyor 122 may include a number of automated
system and/or buffers that operate to autonomously carry workpieces
from the outlet of screen printer 112 to the inlet of placement
machine 114.
[0039] FIG. 8 is a front elevation view of a portion of assembly
line 110. In a preferred embodiment, optical inspection sensors
130, 132, and 134 are configured similarly to optical inspection
sensor 94 shown in FIG. 1. Computer 77 communicates with the
equipment in assembly line 110 and inspection application program
73 computes inspection results using the two-dimensional images
acquired by optical inspection sensors 130, 132, and 134.
Inspection program 73 may also use three dimensional image data to
enhance inspection results when optical inspection sensors 130,
132, and 134 are configured to provide stereo or other three
dimensional image data. The optional/additional three dimensional
image data can be provided for the entire circuit board or selected
regions. Inspection sensors 130, 132, and 134 may be situated in
close proximity to component placement machines 114 and 116 due to
their compact form factors and may be integrated or "embedded"
inside the component placement machines. By utilizing multiple
optical inspection sensors that are distributed throughout the
assembly process, the inspection performance can be improved and
the initial programming of the inspection system can be simplified.
Inputs to inspection program 73 include fiducial reference
indicator locations, and component type, size, location, and
polarity which information is known and available from component
placement machines 114 and 116. Additional information such as
component reference designators, the bar code number of circuit
board 10, as well as the component feeder number, head number, and
nozzle used for a particular component placement are also available
from the component placement machines. Solder paste aperture data
may be inputted into inspection program 73 from screen printer 112
or an external source.
[0040] Inspection application program 73 computes inspection
results for solder paste printing such as print registration, area,
percent coverage, and unintended bridging between adjacent solder
pads. Height and volume may also be computed if three dimensional
image data is available. After components are mounted on circuit
board 10 by component placement machines 114 and 116, inspection
program 73 computes inspection results to verify absence or
presence of a component at a particular location on circuit board
10, whether the correct component was placed, the spatial offset of
a component from its nominal design location, the spatial offset
with respect to the solder paste print, and whether a component was
mounted with the correct polarity. Inspection program 73 also
computes whether a stray component was inadvertently released onto
circuit board 10 at an improper location such as where another
component is to be mounted during a subsequent placement
operation.
[0041] During the assembly process and after solder paste screen
printing, conveyor 122 transports printed circuit board 10 into
component placement machine 114 in a non-stop fashion while
inspection sensor 130 acquires images of circuit board 10 with one
or more illumination field types. These images are transmitted to
computer 77 and are made available to inspection application
program 73 where the solder paste deposits are identified and the
solder paste inspection results are generated.
[0042] Referring back to FIG. 4, it can be seen that circuit board
10 is represented with two duplicate circuits 8 and 9. Circuit
boards are often designed with several duplicate circuits which are
singulated into individual circuits at a later step in the assembly
process. It is common to have eight or more individual circuits on
one circuit board 10. Prior to placing components onto circuit
board 10, component placement machines 114 and 116 must search
circuit board 10 for so-called "bad marks" or "skip marks". These
skip marks identify individual defective circuits for which no
components will be placed. Prior art component placement machines
read the skip marks using a fiducial, or board alignment camera. In
this case, the fiducial camera is positioned over the location of a
potential skip mark. This positioning operation may take 0.5 or
more seconds per skip mark which may add 4 or more seconds to the
placement cycle time for a circuit board with eight individual
circuits, for example.
[0043] In a preferred embodiment, the images acquired by optical
inspection sensors 130 are analyzed by inspection program 73 to
detect the presence or absence of skip marks for individual
circuits. Detected skip marks are communicated by inspection
program 73 to component placement machine 114 and the time
consuming steps of positioning and reading the skip marks with the
fiducial camera are eliminated. In a similar fashion, images
acquired by optical inspection sensor 130 may be analyzed by
inspection program 73 to read the barcodes for circuit board 10 and
individual circuits 8 and 9 and then communicated to component
placement machine 114. This eliminates the expense of a dedicated
barcode reader or the time consuming process of reading the
barcodes with the fiducial camera.
[0044] Component placement machine 114 then places a portion of
electrical components onto circuit board 10. When the assembly
operation by component placement machine 114 is complete, conveyor
124 facilitates transport of circuit board 10 in a non-stop fashion
while optical inspection sensor 132 acquires images of circuit
board 10 with one or more illumination types. These images are
transmitted to computer 77 and are made available to inspection
program 73. Inspection program 73 computes inspection results for
component presence/absence, correct component, spatial offset, and
component polarity for components placed by placement machine 114.
Barcode reading and skip mark detection may also be computed by
inspection program 73 using the images acquired by optical
inspection sensor 132 and the results communicated to component
placement machine 116.
[0045] The component offset with respect to the solder paste
deposits is also computed by inspection program 73 by using images
captured before and after the component placement operation as
explained with respect to FIGS. 9A-9C. FIG. 9A shows example solder
paste deposits 100 and 101 on circuit board 10 identified by
inspection program 73 using the images acquired with optical
inspection sensor 130 before the assembly operation of component
placement machine 114. Local coordinate axes X', Y' are shown that
define the location of the solder paste deposits. FIG. 9B shows an
example image of the same region of circuit board 10 that has been
captured by optical inspection sensor 132 after the assembly
operation of component placement machine 114. Component 15 was
placed on circuit board 10 by component placement machine 114.
Inspection program 73 registers the images captured before and
after the component placement operation and then performs a
difference operation on the registered images. Spatial offsets
.DELTA.X', .DELTA.Y', and .DELTA..theta.' for component 15 are
computed by inspection program 73 using this difference image and
the results are shown in FIG. 9C.
[0046] With the industry trend of electrical component sizes
shrinking ever smaller, there is a risk of component placement
machine 114 inadvertently releasing a component at an improper
location on circuit board 10. For example, if this so-called stray
component was released onto the location where a subsequent ball
grid array (BGA) component was to be mounted by component placement
machine 114, then this error would go undetected by AOI machine 120
since the stray component would not be visible. Circuit board 10
would not function as intended which may result in it being
scrapped, or at least, the faulty BGA site would have to be
diagnosed by other methods and reworked at significant cost.
Inspection program 73 identifies stray components as explained with
respect to FIGS. 10A-10C.
[0047] FIG. 10A shows example image 136 acquired by optical
inspection sensor 130 in the region of where a BGA will be placed
by component placement machine 116. FIG. 10B shows example image
138 acquired by optical inspection sensor 132 in the same region
and where a stray component has been inadvertently released onto
circuit board 10 by component placement machine 114. Inspection
program registers images 136 and 138 and computes the difference
image 140 shown in FIG. 10C. Since no components are intended to be
placed in this region by placement machine 114, the presence of
component in image 140 is an indication of a stray component. The
assembly process may then be halted before additional components
are added to circuit board 10 and additional expense incurred.
Acquiring images 136 and 138 before and after an assembly step
simplifies the initial programming of inspection program 73 since
the difference image segments the stray component from numerous
other valid features.
[0048] When the assembly operation by component placement machine
116 is complete, conveyor 126 facilitates transport of circuit
board 10 in a non-stop fashion while optical inspection sensor 134
acquires images of circuit board 10 with one or more illumination
types. These images are transmitted to computer 77 and are made
available to inspection program 73. Inspection program 73 then
computes inspection results for presence/absence, correct
component, spatial offset, polarity, and offset with respect to the
solder paste deposits for the remaining portion of components
placed onto circuit board 10 by placement machine 116.
[0049] AOI machine 120 computes results such as verifying component
presence/absence, location, polarity, and proper solder joint
fillets after the solder has been reflowed by oven 118. However,
AOI machine 120 cannot identify stray components at BGA or other
larger component sites since they are no longer visible. When AOI
machine 120 does detect an error, it is often difficult to
determine the root cause of an assembly error at that stage in the
assembly process. To facilitate improved root cause failure
analysis, inspection program 73 can provide images of circuit board
10 to the defect review subsystem of AOI machine 120 that were
captured by optical inspection sensors 130, 132, and 134 at the
various stages of the assembly process and in the region of the
defect identified by AOI machine 120. These images help narrow the
list of potential assembly error sources and speed up root cause
failure analysis.
[0050] Although the present invention has been described with
reference to preferred embodiments, workers skilled in the art will
recognize that changes may be made in form and detail without
departing from the spirit and scope of the invention.
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